The cDNA for human IL-6 was stably expressed at high levels in the three mammalian cell lines COS-7, PA317, and GH3 to yield IL-6 proteins of 27- 25, 26, 24-22, and 23 kDa molecular weight. Both size and relative amounts of the recombinant IL-6 species produced correspond to those of natural IL-6 secreted by LPS stimulated monocytes. Oligosaccharide analysis of recombinant IL-6 itilizing tunicamycin and endoglycosidases revealed O and N-linked glycosylation that was comparable to that of natural IL-6 derived from human monocytes and fibroblasts. IL-6 expressed in each of the three cell lines was phosphorylated similar to the IL-6 produced in human monocytes and fibroblasts. IL-6 secreted by each of the three different cell lines revealed marked differences in specific biological activities. COS-7 IL-6 appeared 12 fold more active in its hybridoma growth factor activity than that made in PA317 and GH3 cells. In contrast, PA317 and GH3 IL-6 was 230 and 6.7 times more effective in inducing Ig production in CESS cells when compared to COS-7-IL-6. Also, PA317 and GH3 IL-6 was more effective than COS-7-IL-6 in inducing the acute phase protein fibrinogen in human hepatocytes. The rIL-6 exhibited no antiviral activity.